[build] enable coexistence parameter build (#7540)

This commit enables defining
OPENTHREAD_CONFIG_PLATFORM_RADIO_COEX_ENABLE, which can be used by
external build system (i.e. Matter) to override the platform
defaults. This is useful for platforms having transceivers that also
support coexistence between multiple protocols (WiFi/BLE etc.)`

etc/gn/openthread.gni

@@ -231,6 +231,9 @@ if (openthread_enable_core_config_args) {
     # Enable builtin mbedtls management
     openthread_config_enable_builtin_mbedtls_management =
         openthread_external_mbedtls == ""
+
+    # Enable radio coexistence
+    openthread_config_coexistence_enable = false
   }
 }
 

src/core/BUILD.gn

@@ -264,6 +264,10 @@ if (openthread_enable_core_config_args) {
     if (openthread_config_otns_enable) {
       defines += [ "OPENTHREAD_CONFIG_OTNS_ENABLE=1" ]
     }
+
+    if (openthread_config_coexistence_enable) {
+      defines += [ "OPENTHREAD_CONFIG_PLATFORM_RADIO_COEX_ENABLE=1" ]
+    }
   }
 }